1. Field of the Invention
The present invention relates to a static induction phototransistor type photodetector which has photodetection sensitivity corresponding to a desired spectral responsivity character, and more particularly to a solid-state color image sensor which utilizes the characteristics of the above photodetector.
2. Description of the Related Art
The operation of a photodetector employing a static induction transistor (hereinafter referred to as an SIT) is based on the principles that the gate potential of the SIT is determined by light directed thereto, whereas the potential barrier height in a channel is controlled by a static induction effect of gate and drain voltages. In consequence, the SIT as a photo detecting device features a high current gain, a high signal-to-noise ratio, a wide dynamic range and a high-frequency operation. There have already been proposed by the present inventors SIT photodetectors of various structures (Pat. Appln. No. 192417/81: Pat. Pub. Disc. No. 93386/83). On the other hand, applications have also been made to an image sensor (hereinafter referred to as an SIT image sensor) through utilization of such features of the SIT photodetector as a high current gain (a high optical gain), a high signal-to-noise ratio, a wide dynamic range and a high-frequency operation, and they have already been disclosed by the present inventors in Pat. Appln. No. 87988/78 (Pat. Pub. Disc. No. 15229/80), Pat. Appln. No. 204656/81 (Pat. Pub. Disc. No. 105672/83) and Pat. Appln. No. 157693/82 (Pat. Pub. Disc. No. 45781/84). An attempt to improve the wavelength character, in particular, blue light response on the side of the shorter wavelength in the SIT photodetector or a gate storage type SIT image sensor has already been disclosed in Pat. Appln. No. 217751/82 (Pat. Pub. Disc. No. 108458/84) and Pat. Appln. No. 218926/82 (Pat. Pub. Disc. No. 108472/84). The proposed method is to make structural improvements on the gate section of a pixel of the SIT image sensor. A prior art example is shown in FIG. 1.
FIG. 1 illustrates one pixel of the gate storage type SIT image sensor with increased spectral response for the shorter wavelength, (a) showing the circuit arrangement of the device and (b) its sectional structure. With reference to FIG. 1, respective parts of the device will be described. Reference numeral 10 indicates an n.sup.+ -type substrate or buried layer, which serves as a source region of the SIT section. Reference numeral 11 designates an n.sup.- -, p.sup.- - or i-type high resistivity layer, which forms a channel portion of the SIT. A p.sup.+ -type region 13 formed by a thick diffused region is the gate of the SIT, which is provided for controlling a current flow between an n.sup.+ -type drain 15 and the n.sup.+ -type source 10. A p-type region 12 formed by a relatively thin diffused region is also the gate of the SIT, which is not for current control use but for detecting shorter wavelength light of a relatively small pentration depth. The broken line A shows a depletion layer spreading in the gates 12 and 13, and the broken line B a depletion layer spreading in the high resistivity layer 11. Reference numeral 14 identifies an insulating film of SiO.sub.2 or the like. An electrode 16 serves as a drain electrode and, at the same time, forms a signal readout line. An electrode 17 forms a capacitor between it and the p-type gate 12 with a thin insulating film 18 sandwiched therebetween and, at the same time, serves as a gate address line. Thus an image sensor of the X-Y address system is constituted. As is evident from the spreading of the depletion layer shown in FIG. 1(b), comparison of a pin diode made up of the p.sup.+ -type region 13, the n.sup.- (p.sup.-, i)-type layer 11 and the n.sup.+ -type region 10 and a pin diode made up of the p-type region 12, the n.sup.- (p.sup.-, i)-type layer 11 and the n.sup.+ -type region 10, in terms of photodetection sensitivity, indicates that the p(12) n.sup.- (11)n.sup.+ (10) diode is excellent in the spectral response for the shorter wavelength, whereas the p.sup.+ (13)n.sup.- (11)n.sup.+ (10) diode detects wave light of a relatively large penetration depth.
Sometimes the device of the sectional structure shown in FIG. 1 is used as a discrete element, i.e. as one-element SIT photodetector. In such a case, it may be activated in a normal mode in which the n.sup.+ -type regions 15 and 10 serve as the source and the drain, respectively, or in an inverted mode in which the n.sup.+ -type regions 15 and 10 serve as the drain and the source, respectively.
In order to provide the blue light response, the gate portion of the SIT operating for photodetection is formed by a thin diffused region 12 and the gate portion of the SIT operating for current control of the SIT is formed by the thick diffused region 13, thus detecting the shorter-wavelength light of a small penetration depth by the thin diffused region 12. Futhermore, an example of a gate structure in which a scarcely diffused but depleted region, formed by making a hole locally in the gate portion of the SIT, is provided in place of the abovesaid thin diffused region for the same purpose as described above, has already been proposed in Pat. Appln. No. 218927/82 (Pat. Pub. Disc. No. 108473/84).
In the above prior art example, it is an important point to decrease the diffusion depth of the gate portion so as to raise the spectral response for the shorter wavelength, in particular, the blue light response. With this method, however, it is difficult to constitute the SIT photodetector or a pixel of the image sensor which has a photodetection sensitivity corresponding to a given spectral responsivity character. That is to say, a decrease in the diffusion depth of the p.sup.+ -type gate region of the SIT will surely increase the spectral response for a specific short wavelength, but will make it impossible to control the photodetection sensitivity for the other wavelength ranges. The structure of the prior art example is insufficient especially for the construction of the SIT photodetector or a pixel of this image sensor which has a photodetection sensitivity corresponding to a given spectral responsivity character. Furthermore, the gate structure of the prior art involves only two diffusion processes (a shallow diffusion and a deep diffusion) for its formation and is not limited in the thickness of the high resistivity layer 11 between the gates 12 and 13 and the n.sup.+ -type layer 11. Hence, it is also difficult to achieve a sharp responsivity character for a specific wavelength. Moreover, the prior art example is indefinite in the method of determining specific diffusion depths of the gates, the method of designing the thicknesses of the depletion layers and the limitations on the areas of the gates. In other words, an optimum design from the viewpoint of the spectral responsivity character has not been made in the past.